Astronomers often rely on mass to classify distant objects, most of which have a range of possible masses. Sometimes helpful gaps between ranges make it easy for astronomers to differentiate between similar objects. For example, all known neutron stars are less than 2.1 solar masses (see Physics Today online, September 2019), and all known black holes are more than 5 solar masses. The gap between those two types of compact objects is large enough that there shouldn’t be any ambiguity in distinguishing between them. Now astronomers have discovered a star that violates that mass gap and eludes straightforward classification.
Researchers using the MeerKAT radio telescope in South Africa were searching for millisecond pulsars in the globular cluster NGC 1851 when they found an object that caught their attention: the pulsar J0514–4002E and its mysterious, radio-quiet companion. Ewan Barr (Max Planck Institute for Radio Astronomy) and his research group gathered data on the binary system’s orbit, particularly the rate of change of the periastron—the location in the orbit when the two stars are closest together, which implied an unusually high total-system mass.
Taking additional observations, Barr and colleagues deduced that the system’s total mass is 3.9 solar masses, and they inferred that the mass of the pulsar’s companion was in the range of 2.1–2.7 solar masses. Although that measurement would be reasonable if it was a living star, the companion is too faint to detect, even by the Hubble Space Telescope. Barr and his team concluded that the mystery companion had to be a compact object—either a neutron star or a black hole, despite its mass being in the mass gap between the two.
Whether the companion to J0514–4002E is a black hole or a neutron star is ambiguous. It is the fourth such object found in the mass gap—the others are all progenitors of mergers detected by gravitational waves. Astronomers maybe be able to define those objects once they better understand the physics at work in neutron stars, such as the behavior of matter at densities beyond the stars’ nuclear saturation density.
Unmasking the identity of the companion star will take time. The researchers have not detected any radio pulsations, which would be a clear indication that the object is a massive neutron star. And the data do not provide sufficient clues as to how the binary system formed. Long-term observations taken with more sensitive instruments, like a planned expansion of MeerKAT, would allow the researchers to learn more about the rotation of the companion star; the faster it is spinning, the more likely it is to be a black hole. (E. D. Barr et al., Science 383, 275, 2024.)
